This invention relates to a digital time division, multiplex telecommunications switching system, and more particularly to a tone generator malfunction test implemented by means of a tone generator malfunction detector into which various tones may be switched.
In general, call processing by a telecommunications switching system is accomplished by connecting a terminal, for transmitting and receiving information, to another terminal by means of a switching network. In a digital time division, multiplex telecommunications switching system, the switching network, which includes an information memory, connects one terminal to another terminal "in time" by sequentially storing digital samples of information from first and second terminals in assigned locations of the information memory, swapping the two samples in time and returning the first and second samples respectively to the second and first terminals.
In providing "in time" call processing between analog information terminals, such as telephone instruments, the information transmitted by the terminals must first be sampled (to produce pulse amplitude modulated, PAM, signals) and digitally encoded by analog-to-digital (A/D) sections of code converters. The resulting successive digital samples (multi-bit binary words which represent data viewable as numbers (and which are pulse code modulated, PCM) from each terminal are placed in a particular time channel of successive frames in the switching system. That is, the digital samples, at A/D section outputs, are time multiplexed by multiplexers in order to put several channels of information onto a single transmission path or wire with each channel occupying a dedicated time slot. Whenever a channel is idle, its time slot on the wire will simply be vacant. Time multiplexing of a number of channels serves to minimize the number of physical wires going to the information memories in the switching network. After the swapping has occurred in memory, the information is routed in reverse sequence back through demultiplexers, to digital-to-analog (D/A) sections of code converters and via D/A section outputs to the connected terminal.
The swapping of samples in the information memories is accomplished under computer control. The computer or control complex (abbreviated as "CPU") also keeps a record of the status of the switching system including the state of the terminals so as to properly process a request for service. In a system using the present invention, the computer may be distributed microprocessors as described in Pitroda et al. United States application Ser. No. 842,091 filed Oct. 17, 1977 which is a continuation-in-part of Ser. No. 734,732, filed Oct. 21, 1976, in which microprocessors control call processing and monitor the status of the system.
In connecting one telephone instrument to another, a switching system must provide progress tones, such as dial tones, ring back tones or busy tones, which serve to inform the user of the state of his telephone instrument and the progress of his call. Furthermore, in a switching system that services dual tone, multi-frequency (DTMF) telephone instruments, DTMF tones are required in the system to provide certain maintenance functions. Also other maintenance tones, such as tones used for testing the code converters may be incorporated into the switching system.
A switching system may require that a large number of different tones be made available (eighteen or more is not uncommon), each tone being provided by a different tone generator or all tones coming from a time-shared universal source as disclosed in United States application Ser. No. 931,876, filed Aug. 8, 1978 in the names of Mehta et al and assigned to the assignee of the present application. One form of digital tone generator is disclosed in U.S. Pat. No. 3,706,855 issued Dec. 19, 1972 in the names of Pitroda and Rekiere and which confirms that it is known in the art to send tones by outputting in seriatum order, during each of successive frames, the pre-formed PCM digital words of cyclic analog waves. The various tones, in terms of common characteristics, fall into two categories--continuous tones (e.g., dial tones and DTMF tones) and interrupted tones (e.g., ring back tones and busy tones), the latter having alternate on/off intervals of sound and quiet.
A malfunction of a digital tone generator, or the source of any tone, presents a unique maintenance problem within the switching system. While the tone generators are used on a system-wide basis, the failure of a tone generator may not have a calamitous effect on the switching system's performance. For example, the failure of a maintenance tone generator would have no effect at all on the system's immediate ability to process calls, but could create hours of wasted time in attempts to diagnose troubles on the assumption the correct maintenance or test tone was being used. The failure of a progress tone generator would only effect calls being initiated, but would have no effect on those already in progress. In that regard, the failure of a dial tone would be immediately apparent to a user placing a call, and undoubtedly a user complaint would result.
Keeping in mind that telephone switching equipment must have an extremely high degree of reliability, with a design goal of one hour of system down time in twenty years, it is indeed important to be able to verify that all of many tones are correctly available when needed in the system.
It is the general aim of the present invention to do away with apparatus uniquely assigned to test each of the many tones available in a PCM multiplex communication system, and to provide simple, common methods and apparatus for selectively testing each one of many digital tone signals to detect malfunctions in the tone generator equipment.
An object of coordinate significance is to provide common methods and apparatus by which malfunctions or faults in various ones of several digital tone signals may be detected, despite the fact that some tone signals represent continuous sounds and others are turned on and off to represent interrupted sounds.
It is also an object of the present invention to provide methods and apparatus to test tone signals and detect malfunctions therein by initiation commands from a central control (the CPU); so that some or all of the tone signals may be checked when, and as often as, it is desired.
Another object is to monitor any of a large plurality of tone generator tones by common test apparatus by the efficient procedure of routing any of the tones through a connection set up in a switching network, to the input of a single error detector device.
A specific object of the invention is to provide a common, simple test for monitoring any one of a large plurality of tones, regardless of its characteristics, but which by itself might make the test results inaccurate for interrupted tones, while providing a second common test procedure to remove that possible inaccuracy without detracting from the reliability of tests on continuous tones.
Yet another object of the invention is to provide methods and apparatus for detecting malfunctions or errors in tone generators or tone signals and further to produce signals which will indicate when the procedures or the detecting apparatus are themselves faulty.